Generally you need to fit wheels/tyres that were fitted by the manufacturer to a model in the range.

Examples:

Our 118D has 205/50 17 V from the factory, 205/55 16 V were fitted to other models, insurance happy for us to fit these on OEM wheels, no charge.

Our Kia Ceed has 225/45/17 W form factory, 205/55 16 V were fitted to other models, insurance happy for us to fit these on OEM wheels, no charge.

When I originally asked about using steel wheels there would have been an extra premium to pay since they were not a standard fitment on our models, afater a quick search on e-bay I bought alloys for much less than steels.

The basis rule is inform the insurance company what you intend doing, if they are happy you are OK.

I have a 2009 Mondeo titanium x with 18" wheels.The size is 235/40 r18.

I have fitted 16" winter wheels for the past 2 years now with a tyre size of 205/55r16.

They fit fine over the brakes and are far superior on ice and snow.The 18" summer tyres are useless in winter and before buying the winter wheels i skidded across the road and ended up facing oncoming traffic.Luckily nothing was coming at the time but it shook me up somewhat.

Since fitting the winter tyres i can now drive up very steep hills on snow without any fuss where as the summer tyres slipped even on the flat.

I would highly recommend fitting winter tyres if your summer ones are of the wide low profile type.

Thanks Videodoctor, sounds like we have a plan.